I've decided to make some changes to my approach with the boot soles. There are limitations to what I can accomplish with Bondo, so I'm going to do a hybrid approach. My plan will be to do the general sole shape and bulk with bondo, but do surface details with Apoxie Sculpt. This is a relatively new product that is a combination of clay and epoxy. It's a two part putty that you mix together, then handle like clay. But it cures solid in 24 hours. This will allow me to do the delicate and difficult sculpting tasks with a clay-like media, then once it hardens I can sand it and file it like bondo. Should be good!
I ordered a 4 pound kit of white Apoxie Sculpt from ebay today.